yes and no... some things can be changed, some can't -- what exactly do you want to do?
If you want to have three partitions (or three hard drives) in one machine, and have them C, D & G with no E & F... I'm not sure how that can be done...
you can play with what letter your CD-ROM/DVD/Burner is assigned to etc, but I think those come in effect after the hard drive/partitions take the first available letters...
One way is to make small partitions (say a couple of megabytes), have them be in NTFS, and put those as E & F... I don't think you'd see them in Win9x/ME, but they would still be reserved and you could have your jump from D to G... that's WAY too messy though